During endless tennis, universal boredom (5)
I believe the answer is:
ennui
'boredom' is the definition.
('ennui' can be a synonym of 'boredom')
'during endless tennis universal' is the wordplay.
'during' is an insertion indicator.
'endless' indicates the central letters.
'universal' becomes 'u'.
The centre of 'tennis' is 'enni'.
'enni' placed around 'u' is 'ENNUI'.
(Other definitions for ennui that I've seen before include "boring" , "tiredness" , "The feeling of being bored" , "Boredom, listlessness" , "Sense of tedium" .)